During an interview on Fox News, Mr Mulvaney was asked if Congress would ever see the president’s tax returns.

“No, never,” he replied. “Nor should they.”

He added: “Democrats are demanding that the IRS turn over the documents. That is not going to happen, and they know it. This is a political stunt.”

In response, Democrats said that the request by tax committee chairman Bill Neal was both legal and necessary.

Dan Kildee, a Democrat and a tax committee member, told ABC: “This is a legitimate authority that the Congress has. This president, by the way, is the least transparent president we’ve had in half a century.”

Every US presidential candidate since 1976 has released their tax returns, but there is no law requiring it.

The Democrats gained control of the House of Representatives in mid-term elections last year, giving them the ability to launch investigations into Mr Trump’s administration and business affairs.

It comes after one of Mr Trump’s lawyers said it was “harassment” to keep asking for his tax returns, and hinted at possible legal action in the future.

“His request is a transparent effort by one political party to harass an official from the other party because they dislike his politics and speech,” attorney William Consovoy said of Mr Neal.

He said the request was a “misguided attempt” to politicise tax laws and could also end up interfering with audits. He said the US Treasury should not comply with the demand.

Mr Trump also added that he believed the law was “100% on my side”.

Mr Trump has in the past said that he is unable to release his tax returns because they were being audited by the Internal Revenue Service (IRS).

However, the IRS has said that he could release the returns even if they are under audit.

In February Mr Trump’s former lawyer Michael Cohen suggested during testimony to Congress that Mr Trump’s taxes were not under audit during the 2016 presidential campaign – when Mr Trump said they were.

Mr Trump had not wanted to release the tax returns because the resulting scrutiny could have led to an audit and “he’ll ultimately have taxable consequences, penalties and so on”.

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